1. Field of the Invention
This invention relates to a low power light pen interface unit which correlates signals from a light pen with a position on a cathode ray tube video monitor used in conjunction with a computer.
2. Description of the Prior Art
Information is provided to a computer in a number of ways. One way of providing information is by direct input from a user. The most common user interface for inputting information is the keyboard. The keyboard requires the user to type information in the same manner as a typist with a typewriter. However, this is not the only was a user can provide information.
Another method of providing information to the computer relies upon the relative position of a cursor to a location on a video display such as a cathode ray tube (CRT) . This can be accomplished by use of a mouse, light pen or other conventional devices. A mouse is a device that rests on a pad next to the computer and generally includes a ball which responds to the movement of the mouse by the user's hand in an x-y plane. This motion is translated onto the CRT by way of a cursor which points to a specific location. When positioned correctly, the user may use a button, or buttons, on the mouse to direct the computer to perform an action dictated by the cursor's location. That action can take many forms. Three such options are; 1) to issue a command to perform some function, or; 2) request information, or; 3) select from a menu of available options.
A light pen is similar in appearance to a pen or pencil-type writing instrument. It differs from the mouse in that the tip of the light pen when placed in close proximity with the surface of the CRT receives light signals, via a photo detector, emanating from the illumination of the phosphorous coating as the electron gun within the CRT scans the pixels on the internal surface of the CRT. Light pens interact directly with the video monitor's screen like a touchscreen, in providing the absolute positioning and pen-type input of a digitizing tablet, but do not suffer from the confusion and ambiguity of a relative positioning mouse. For most operating systems light pens are used as pointing devices with the same basic functional characteristics (to the operating system) as a mouse. Special device driver software supplied by the light pen manufacturer provides the light pen with these characteristics. The light pen is typically provided with a button, or buttons, that allow the user to execute a number of options in a manner similar to the buttons on a mouse.
Light pens are preferred over a mouse by many users. One reason for this is that the light pen does not require a pad and thus occupies less space on a desk. Another reason is that the use of a light pen for drawing is more comfortable as it is closely aligned with the method by which most people learn to draw. That is, with a pen, pencil, or brush.
Prior art light pen systems have suffered from a number of disadvantages. To determine the position of a light pen on the face of a CRT video monitor it is necessary to synchronize the receipt of the signal with the video synchronization ("sync") signals i.e. the horizontal and vertical sync signals of the CRT raster. In the prior art systems it was necessary to make physical interconnection with the video sync circuitry inside of the monitor in order to obtain the necessary sync signals. Such an interconnection required time and some expertise.
In addition to the physical video sync circuitry interconnection, prior art light pens required an external power source to supply the power utilized by the digital timing circuitry needed for the precise timing measurements between the light pen signal and the CRT's sync signals. Typically a high frequency oscillator (to provide the necessary precision) and a counter was used for such measurements. However, such high frequency digital circuitry required considerable power e.g. of the order of 400+ milliwatts for the horizontal measurement alone. This high power requirement, as a practical matter, ruled out the use of the computer power supply as the source, thereby requiring an external power supply. In addition to the high power requirements the prior art all digital timing circuitry utilizing a fixed frequency oscillator, suffered from decreasing resolution as the CRT scanning frequency increased to provide a higher resolution. Such a fixed frequency approach provides less time to accumulate counts with an increased horizontal scanning frequency.
In addition to the above shortcomings of prior art light pens the market for such is target specific. For example, if the target marketplace is PC-compatible computers running Microsoft Windows 95 as an operating system, then the manufacturer must develop not only a compatible interface for the light pen, but also develop device driver software to support the interface and make it functional within that operating system. This approach has the following disadvantages for the Lightpen Manufacturer:
(a) It requires the manufacturer's software developer to have detailed knowledge of the inner workings of each of the target operating systems; PA1 (b) It is prone to bugs and incompatibilities that generally aren't discovered through in-house testing. The manufacturer, as a practical matter, cannot possibly recreate all of the system configuration combinations that are in use in the marketplace and; PA1 (c) It is subject to problems caused by periodic changes to the operating system made by it's producer.
This development effort must be repeated by the manufacturer for each target operating system and computer type, a costly and time-consuming process.
There is a need for a light pen interface system which overcomes the above shortcomings of the prior art.